1. Field of the Invention
The present invention relates to an information recording method which determines a recording waveform of a light beam at a time of recording information by irradiating light to an optical disc and records the information using the light beam, an information recording apparatus which records information on an optical information recording medium, an information recording medium and a medium evaluating method.
2. Background Art
Currently, as an optical disc that is an optical information recording medium, a CD (Compact Disc), a DVD (Digital Versatile Disc), a BD (Blu-ray Disc), a BDXL™, and the like have been commercialized and widely spread in use. These optical discs include various kinds of discs such as a ROM (Read Only Memory) type which is dedicated for reproduction, a R (Recordable) type which is a re-recordable type, a RE (Rewritable) type which is a rewritable type.
A laser beam is irradiated to the optical disc to perform information recording and reading on the optical disc. An area is formed in which a state of a recording film material is changed by heat of the laser beam, whereby information recording is performed. An area where the state is changed is referred to as a mark, and another area where the state is not changed is referred to as a space. As the recording film, a phase-change material, an organic dye, and an alloy and an oxide of inorganic material are used. A mark edge recording is used as an encoding method in the CD, the DVD, the BD, and the BDXL™, and information is recoded as an edge position of a mark.
In the recording, it is necessary to control the heat that is generated by irradiation of a laser beam so as to form a mark of any length in the recording film. Therefore, the laser beam at a time of recording is controlled in various waveforms depending on a mark length to be formed. FIGS. 1A and 1B are examples of laser emitting waveforms to be used when marks of 2T to 5T (T is a channel bit length) are formed. These are recording waveforms to be used in BDXL™ specification, FIG. 1A is referred to as an N-1 strategy, and FIG. 1B is referred to as a castle strategy. In the N-1 strategy, a mark of a length of NT is recorded using (N-1) pulses. The first pulse in a pulse train is referred to as a first pulse, and the last pulse in the pulse train is referred to as a last pulse. The pulse between the first pulse and the last pulse is referred to as a multi-pulse. In a case of 4T mark, the number of the multi-pulse is one. The number of the multi-pulse is increased by one, as the subsequent mark length is increased by 1T. A pulse of 2T mark has only the first pulse. A pulse of 3T mark has only the first pulse and the last pulse. Both the pulses of 2T mark and 3T mark do not respectively include the multi-pulse. On the other hand, in the castle strategy, each waveform is mainly configured of either only first pulse or both of the first pulse and the last pulse without the multi-pulse, and the waveform between the first pulse and the last pulse is kept at a constant power. Since a pulse having a short width is not used in the waveform, the waveform is suitable for recording at high speed.
With respect to a laser beam power in each strategy, the N-1 strategy uses four kinds of powers, that is, a recording power (Pw), a space power (Ps), a bias power (PBW), and a cooling power (Pc), whereas the castle strategy uses four kinds of powers, that is, an intermediate power (Pm), in addition to the Pw, the Ps, and the Pc. Here, in the RE type optical disc, the Ps is referred to as an erase power (Pe). The Pw has a maximum power level in a pulse train, and is used to mainly cause a state change by inputting energy to the recording film. The Ps is the power level to be irradiated to a part that becomes a space, and is used mainly in preheating to form a next mark. The Pe in the RE type optical disc plays a role to return the mark that is already recorded to a space, in addition to the preheating. The Pc is the power level right after the last pulse, and mainly plays a role to block heat diffusion to the subsequent mark recording part in the R type optical disc, and a role to rapidly cool the recording film in order to form the mark in the RE type optical disc.
Although the recording of the optical disc is performed using the aforementioned recording waveforms, the recording waveforms are different depending on the type of the optical disc and the recording layer. This is resulted from that the material of the recording film and the formation mechanism of a mark are different. Therefore, the recording waveform is prepared for each type of the optical disc and recording layer. As indexes for defining the recording waveform, there are dTtop, Ttop, TMP, dTLP, TLP, dTc, dTS, and dTE for defining the pulse width, and the like, in addition to the aforementioned various power levels Pw, Pm, Ps, Pe, Pc, and PBW. The dTtop indicates the start end position of the first pulse, the Ttop indicates the time width of first pulse, the TMP indicates the time width of the multi-pulse, the dTLP indicates the start end position of the last pulse, the TLP indicates the time width of the last pulse, the dTS indicates the start position of the Ps, and the dTE indicates the start position of the Pe. These parameters are set in 1/16 unit of the channel bit according to the BD specification, and are set in 1/32 unit of the channel bit according to BDXL™ specification.
The recording waveform in each optical disc and the recording layer is mainly prepared by an optical disc maker, and the determined recording waveform is converted into the index of the recording waveform and is recorded in the management region of the optical disc. Accordingly, in a case of recording information on the optical disc using a drive that is an optical information recording and reading apparatus, the drive obtains the recording waveform of the optical disc from management information on the optical disc, and thus starts the recording without adjustment of the recording waveform. Here, even in the same type of optical disc, there may be a difference in an optimum recording waveform for each optical disc, especially, in a power level of a laser due to variations in production. Further, even in a case of using the same optical disc, the optimum recording may not be performed in the same recording waveform, especially in the power level of the same laser, due to the individual differences of drives. In order to correspond to the individual differences of the optical discs and the drives, the drive has means for adjusting the recording waveform. As one of adjustment methods, there is a method to optimize the laser power level called OPC (Optimum Power Control). This enables to realize a proper recording without being affected by the individual differences of the optical discs and the drives.
The adjustment of the recording waveform in the aforementioned optical disc maker and the drive is performed by evaluating the quality of the recording signal in each recording waveform. As evaluation indexes of the recording signal, there are a Jitter of evaluating the aged fluctuation in the mark edge, bER (bit Error Rate) of evaluating the decoded result of a reading signal, SER (Symbol Error Rate), and the like. Further, as an index of evaluating the reading signal quality based on PRML to be used in reading signal processing of BDLX™, i-MLSE (Integrated-Maximum Likelihood Sequence Error Evaluation) is disclosed in Pamphlet of International Publication WO2010-001588. i-MLSE is an index of statistically evaluating an error amount of a target waveform and a reading signal waveform that are predicted at a time of decoding. Besides this, SbER (Simulated bit Error Rate) is disclosed in JP-A-2004-253114. SbER is an index of quantifying an estimation value of an error probability from an error amount of the target waveform and the reading signal waveform. Furthermore, L-SEAT (run-length-Limited Sequence Error for Adaptive Target) which is an index of evaluating a shift of an edge position of a recording mark is disclosed in US 2010-0260025. L-SEAT is a value that is obtained by calculating the error amount of the reading signal waveform and Left target waveform and Right target waveform that are assumed in a case where the edge is shifted in the left or right with respect to the target waveform, before and after each edge of each mark length. Since the shift amount of the mark edge can be evaluated using L-SEAT, L-SEAT is suggested as an adjustment index of the recording waveform in BDXL™ specification.